In proteins that have segments extending extracellularly, the extracellular segments are also often glycosylated. Glycoproteins are also often important integral membrane proteins, where they play a role in cell–cell interactions. It is important to distinguish endoplasmic reticulum-based glycosylation of the secretory system from reversible cytosolic-nuclear glycosylation. Glycoproteins of the cytosol and nucleus can be modified through the reversible addition of a single GlcNAc residue that is considered reciprocal to phosphorylation and the functions of these are likely to be additional regulatory mechanism that controls phosphorylation-based signalling.[2] In contrast, classical secretory glycosylation can be structurally essential. For example, inhibition of asparagine-linked, i.e. N-linked, glycosylation can prevent proper glycoprotein folding and full inhibition can be toxic to an individual cell. In contrast, perturbation of glycan processing (enzymatic removal/addition of carbohydrate residues to the glycan), which occurs in both the endoplastic reticulum and Golgi apparatus, is dispensable for isolated cells (as evidence by survival with glycosides inhibitors) but can lead to human disease (congenital disorders of glycosylation) and can be lethal in animal models. It is therefore likely that the fine processing of glycans is important for endogenous functionality, such as cell trafficking, but that this is likely to have been secondary to its role in host-pathogen interactions. A famous example of this latter effect is the ABO blood group system.

The sugar group(s) can assist in protein folding, improve proteins' stability and are involved in cell signalling.

Examples

One example of glycoproteins found in the body is mucins, which are secreted in the mucus of the respiratory and digestive tracts. The sugars when attached to mucins give them considerable water-holding capacity and also make them resistant to proteolysis by digestive enzymes.

structural glycoproteins, which occur in connective tissue. These help bind together the fibers, cells, and ground substance of connective tissue. They may also help components of the tissue bind to inorganic substances, such as calcium in bone.

The viral spike of the human immunodeficiency virus is heavily glycosylated.[7] Approximately half the mass of the spike is glycosylation and the glycans act to limit antibody recognition as the glycans are assembled by the host cell and so are largely 'self'. Over time, some patients can evolve antibodies to recognise the HIV glycans and almost all so-called 'broadly neutralising antibodies (bnAbs) recognise some glycans. This is possible mainly because the unusually high density of glycans hinders normal glycan maturation and they are therefore trapped in the premature, high-mannose, state.[8][9] This provides a window for immune recognition. In addition, as these glycans are much less variable than the underlying protein, they have emerged as promising targets for vaccine design.[10]

In conjunction with size-exclusion chromatography, UV/Vis absorption and differential refractometry, provides information on molecular mass, protein-carbohydrate ratio, aggregation state, size, and sometimes branching of a glycan chain. In conjunction with composition-gradient analysis, analyzes self- and hetero-association to determine binding affinity and stoichiometry with proteins or carbohydrates in solution without labeling.

Abciximab or Abcixifiban (previously known as c7E3 Fab), a glycoprotein IIb/IIIa receptor antagonist manufactured by Janssen Biologics BV and distributed by Eli Lilly under the trade name ReoPro, is a platelet aggregation inhibitor mainly used during and after coronary artery procedures like angioplasty to prevent platelets from sticking together and causing thrombus (blood clot) formation within the coronary artery. It is a glycoprotein IIb/IIIa inhibitor.While abciximab has a short plasma half-life, due to its strong affinity for its receptor on the platelets, it may occupy some receptors for weeks. In practice, platelet aggregation gradually returns to normal about 96 to 120 hours after discontinuation of the drug. Abciximab is made from the Fab fragments of an immunoglobulin that targets the glycoprotein IIb/IIIa receptor on the platelet membrane.

In molecular biology, CD4 (cluster of differentiation 4) is a glycoprotein found on the surface of immune cells such as T helper cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 (after the OKT4 monoclonal antibody that reacted with it) before being named CD4 in 1984. In humans, the CD4 protein is encoded by the CD4 gene.CD4+ T helper cells are white blood cells that are an essential part of the human immune system. They are often referred to as CD4 cells, T-helper cells or T4 cells. They are called helper cells because one of their main roles is to send signals to other types of immune cells, including CD8 killer cells, which then destroy the infectious particle. If CD4 cells become depleted, for example in untreated HIV infection, or following immune suppression prior to a transplant, the body is left vulnerable to a wide range of infections that it would otherwise have been able to fight.

Carcinoembryonic antigen-related cell adhesion molecule 1 (biliary glycoprotein) (CEACAM1) also known as CD66a (Cluster of Differentiation 66a), is a human glycoprotein, and a member of the carcinoembryonic antigen (CEA) gene family.

Envelope glycoprotein GP120 (or gp120) is a glycoprotein exposed on the surface of the HIV envelope. It was discovered by Professors Tun-Hou Lee and Myron "Max" Essex of the Harvard School of Public Health in 1988. The 120 in its name comes from its molecular weight of 120 kDa. Gp120 is essential for virus entry into cells as it plays a vital role in attachment to specific cell surface receptors. These receptors are DC-SIGN, Heparan Sulfate Proteoglycan and a specific interaction with the CD4 receptor, particularly on helper T-cells. Binding to CD4 induces the start of a cascade of conformational changes in gp120 and gp41 that lead to the fusion of the viral membrane with the host cell membrane. Binding to CD4 is mainly electrostatic although there are van der Waals interactions and hydrogen bonds.

Gp120 is coded by the HIV env gene, which is around 2.5 kb long and codes for around 850 amino acids. The primary env product is the protein gp160, which gets cleaved to gp120 (~480 amino acids) and gp41 (~345 amino acids) in the endoplasmatic reticulum by the cellular protease furin. The crystal structure of core gp120 shows an organization with an outer domain, an inner domain with respect to its termini and a bridging sheet. Gp120 is anchored to the viral membrane, or envelope, via non-covalent bonds with the transmembrane glycoprotein, gp41. Three gp120s and gp41s combine in a trimer of heterodimers to form the envelope spike, which mediates attachment to and entry into the host cell.

Glycoprotein V (platelet) (GP5) also known as CD42d (Cluster of Differentiation 42d), is a human gene.Human platelet glycoprotein V (GP5) is a part of the Ib-V-IX system of surface glycoproteins that constitute the receptor for von Willebrand factor (VWF; MIM 193400) and mediate the adhesion of platelets to injured vascular surfaces in the arterial circulation, a critical initiating event in hemostasis. The main portion of the receptor is a heterodimer composed of 2 polypeptide chains, an alpha chain (GP1BA; MIM 606672) and a beta chain (GP1BB; MIM 138720), that are linked by disulfide bonds. The complete receptor complex includes noncovalent association of the alpha and beta subunits with platelet glycoprotein IX (GP9; MIM 173515) and GP5. Mutations in GP1BA, GP1BB, and GP9 have been shown to cause Bernard-Soulier syndrome (MIM 231200), a bleeding disorder.[supplied by OMIM]

Thus, the two substrates of this enzyme are UDP-galactose and glycoprotein N-acetyl-D-galactosamine, whereas its two products are UDP and glycoprotein D-galactosyl-1,3-N-acetyl-D-galactosamine.

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is UDP-galactose:glycoprotein-N-acetyl-D-galactosamine 3-beta-D-galactosyltransferase. This enzyme is also called uridine diphosphogalactose-mucin beta-(1->3)-galactosyltransferase. This enzyme participates in o-glycan biosynthesis and glycan structures - biosynthesis 1.

In medicine, glycoprotein IIb/IIIa (GPIIb/IIIa, also known as integrin αIIbβ3) is an integrin complex found on platelets. It is a receptor for fibrinogen and von Willebrand factor and aids platelet activation. The complex is formed via calcium-dependent association of gpIIb and gpIIIa, a required step in normal platelet aggregation and endothelial adherence. Platelet activation by ADP (blocked by clopidogrel) leads to the aforementioned conformational change in platelet gpIIb/IIIa receptors that induces binding to fibrinogen. The gpIIb/IIIa receptor is a target of several drugs including abciximab, eptifibatide, and tirofiban.

Glycoprotein IX (platelet) (GP9) also known as CD42a (Cluster of Differentiation 42a), is a human gene.Platelet glycoprotein IX (GP9) is a small membrane glycoprotein found on the surface of human platelets. It forms a 1-to-1 noncovalent complex with glycoprotein Ib (GP Ib), a platelet surface membrane glycoprotein complex that functions as a receptor for von Willebrand factor (VWF; MIM 193400) (known as the Glycoprotein Ib-IX-V Receptor Complex). The main portion of the receptor is a heterodimer composed of 2 polypeptide chains, an alpha chain (GP1BA; MIM 606672) and a beta chain (GP1BB; MIM 138720), that are linked by disulfide bonds. The complete receptor complex includes noncovalent association of the alpha and beta subunits with GP9 and platelet glycoprotein V (GP5; MIM 173511).[supplied by OMIM]

is a component of the GPIb-V-IX complex on platelets. The GPIb-V-IX complex binds von Willebrand factor, allowing platelet adhesion and platelet plug formation at sites of vascular injury.

It is deficient in the Bernard-Soulier syndrome. A gain-of-function mutation causes platelet-type von Willebrand's disease.Autoantibodies against Ib/IX can be produced in immune thrombocytopenic purpura.Components include GP1BA and GP1BB.

Gonadotropins are glycoprotein polypeptide hormones secreted by gonadotrope cells of the anterior pituitary of vertebrates. This family includes the mammalian hormones follicle-stimulating hormone (FSH), luteinizing hormone (LH), and placental/chorionic gonadotropins, human chorionic gonadotropin (hCG) and equine chorionic gonadotropin (eCG), as well as at least two forms of fish gonadotropins. These hormones are central to the complex endocrine system that regulates normal growth, sexual development, and reproductive function. LH and FSH are secreted by the anterior pituitary gland, while hCG and eCG are secreted by the placenta in pregnant humans and mares, respectively. The gonadotropins act on the gonads, controlling gamete and sex hormone production.

Gonadotropin is sometimes abbreviated Gn. The alternative spelling gonadotrophin which inaccurately implies a nourishing mechanism is still sporadically used.

There are various preparations of gonadotropins for therapeutic use, mainly as fertility medication. There are also fad diet or quack preparations, which are illegal in various countries.

Herpes simplex virus 1 and 2 (HSV-1 and HSV-2), also known as human herpesvirus 1 and 2 (HHV-1 and HHV-2), are two members of the human Herpesviridae family, a set of viruses that produce viral infections in the majority of humans. Both HSV-1 (which produces most cold sores) and HSV-2 (which produces most genital herpes) are very common and contagious. They can be spread when an infected person begins shedding the virus. About 67% of the world population under the age of 50 has HSV-1. In the United States more than one-in-six people have HSV-2. Although it can be transmitted through any intimate contact, it is one of the most common sexually transmitted infections.Many of those who are infected never develop symptoms. Symptoms, when they occur, may include watery blisters in the skin or mucous membranes of the mouth, lips, nose, or genitals. Lesions heal with a scab characteristic of herpetic disease. Sometimes, the viruses cause very mild or atypical symptoms during outbreaks. However, they can also cause more troublesome forms of herpes simplex. As neurotropic and neuroinvasive viruses, HSV-1 and -2 persist in the body by hiding from the immune system in the cell bodies of neurons. After the initial or primary infection, some infected people experience sporadic episodes of viral reactivation or outbreaks. In an outbreak, the virus in a nerve cell becomes active and is transported via the neuron's axon to the skin, where virus replication and shedding occur and cause new sores.

P-glycoprotein 1 (permeability glycoprotein, abbreviated as P-gp or Pgp) also known as multidrug resistance protein 1 (MDR1) or ATP-binding cassette sub-family B member 1 (ABCB1) or cluster of differentiation 243 (CD243) is an important protein of the cell membrane that pumps many foreign substances out of cells. More formally, it is an ATP-dependent efflux pump with broad substrate specificity. It exists in animals, fungi, and bacteria, and it likely evolved as a defense mechanism against harmful substances.

P-gp is extensively distributed and expressed in the intestinal epithelium where it pumps xenobiotics (such as toxins or drugs) back into the intestinal lumen, in liver cells where it pumps them into bile ducts, in the cells of the proximal tubule of the kidney where it pumps them into urinary filtrate (in the proximal tubule), and in the capillary endothelial cells composing the blood–brain barrier and blood-testis barrier, where it pumps them back into the capillaries.

P-gp is a glycoprotein that in humans is encoded by the ABCB1 gene. P-gp is a well-characterized ABC-transporter (which transports a wide variety of substrates across extra- and intracellular membranes) of the MDR/TAP subfamily. The normal excretion of xenobiotics back into the gut lumen by P-gp pharmacokinetically reduces the efficacy of some pharmaceutical drugs (which are said to be P-gp substrates). In addition, some cancer cells also express large amounts of P-gp, further amplifying that effect and rendering these cancers multidrug resistant. Many drugs inhibit P-gp, typically incidentally rather than as their main mechanism of action; some foods do as well. Any such substance can sometimes be called a P-gp inhibitor.

Selectin P ligand, also known as SELPLG or CD162 (cluster of differentiation 162), is a human gene.

SELPLG codes for PSGL-1, the high affinity counter-receptor for P-selectin on myeloid cells and stimulated T lymphocytes. As such, it plays a critical role in the tethering of these cells to activated platelets or endothelia expressing P-selectin.

The organization of the SELPLG gene closely resembles that of CD43 and the human platelet glycoprotein GpIb-alpha both of which have an intron in the 5-prime-noncoding region, a long second exon containing the complete coding region, and TATA-less promoters.P-selectin glycoprotein ligand-1 (PSGL-1) is a glycoprotein found on white blood cells and endothelial cells that binds to P-selectin (P stands for platelet), which is one of a family of selectins that includes E-selectin (endothelial) and L-selectin (leukocyte). Selectins are part of the broader family of cell adhesion molecules. PSGL-1 can bind to all three members of the family but binds best (with the highest affinity) to P-selectin.

Rh-associated glycoprotein (RHAG) is an ammonia transporter protein that in humans is encoded by the RHAG gene. RHAG has also recently been designated CD241 (cluster of differentiation 241). Mutations in the RHAG gene can cause stomatocytosis.

The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a glycoprotein layer surrounding the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the ovarian follicles. The zona pellucida is surrounded by the cumulus oophorus. The cumulus is composed of cells that care for the egg when it is emitted from the ovary.This structure binds spermatozoa, and is required to initiate the acrosome reaction.

In the mouse (the best characterised mammalian system), the zona glycoprotein, ZP3, is responsible for sperm binding, adhering to proteins on the sperm plasma membrane (GalT). ZP3 is then involved in the induction of the acrosome reaction, whereby a spermatozoon releases the contents of the acrosomal vesicle. The exact characterisation of what occurs in other species has become more complicated as further zona proteins have been identified.In humans, five days after the fertilization, the blastocyst performs zona hatching; the zona pellucida degenerates and decomposes, to be replaced by the underlying layer of trophoblastic cells.

The zona pellucida is essential for oocyte growth and fertilization.

In some older texts, it has also been called zona striata and stratum lucidum (not to be confused with the stratum lucidum of the skin).

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